1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for improving a coupling between the ocean bottom and an autonomous underwater vehicle (AUV) that carries seismic sensors for a marine seismic survey.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in determining the above-noted reservoirs. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections and/or refractions to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off geological structures present in the subsurface includes a vessel that tows an array of seismic receivers provided on streamers. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface, or they may have other than horizontal spatial arrangements. The vessel also tows a seismic source array configured to generate a seismic wave. The seismic wave propagates downward and penetrates the seafloor until eventually a reflecting structure (reflector) reflects the seismic wave. The reflected seismic wave propagates upward until detected by the receiver(s) on the streamer(s). Based on the data collected by the receiver(s), an image of the subsurface is generated.
However, this traditional configuration is expensive because the cost of streamers is high. Further, this configuration is limited when various obstacles (e.g., a rig) are present in the surveying area. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to improve the coupling. Even so, positioning seismic sensors remains a challenge.
Other technologies use permanent receivers set on the ocean bottom, as disclosed in U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference. In this case, the seismic sensors are attached to a heavy pedestal. A station that includes the sensors is launched from a vessel and arrives, due to its gravity, at a desired position and remains on the bottom of the ocean permanently. Data recorded by sensors is transferred through a cable to a mobile station. When necessary, the mobile station may be brought to the surface to retrieve the data.
Although this method provides better coupling between the ocean bottom and the sensors, it is still expensive and not flexible because the sensors and corresponding pedestals are left on the seafloor. Further, positioning the sensors is not straightforward.
A different approach is the use of plural AUVs for carrying seismic sensors and collecting seismic data. The AUVs may be (1) launched from a deployment vessel, (2) guided to a final destination on the ocean bottom, (3) instructed to record seismic data, and then (4) instructed to surface or dock to a receiving vessel for transmitting the seismic data. However, deploying AUVs for collecting seismic data poses many challenges, such as the coupling between the ocean bottom and the seismic sensor. The seismic sensor is currently located on the AUV's outer skin or in a chamber inside the AUV, so it is possible for the seismic sensor to not come in direct contact with the ocean bottom. Further, if the ocean bottom is hard, the AUV itself may not have good contact with it. If marine currents are present, the AUV may drift from its intended target location, which degrades the recorded seismic data.
Accordingly, it would be desirable to provide systems and methods that provide an inexpensive and simple way to achieve good coupling between the AUV and the ocean bottom.